[HLSL] Remove variables that are used only in assert (#107299)

Changes the assert to test the same condition without using the
variables.

This change is done in response to a comment
[here](#106657 (comment)).

Author: hekota

clang/lib/Sema/SemaHLSL.cpp

@@ -837,17 +837,10 @@ static void ValidateMultipleRegisterAnnotations(Sema &S, Decl *TheDecl,
 static void DiagnoseHLSLRegisterAttribute(Sema &S, SourceLocation &ArgLoc,
                                           Decl *TheDecl, RegisterType regType) {
 
-  // Samplers, UAVs, and SRVs are VarDecl types
-  VarDecl *TheVarDecl = dyn_cast<VarDecl>(TheDecl);
-  // Cbuffers and Tbuffers are HLSLBufferDecl types
-  HLSLBufferDecl *CBufferOrTBuffer = dyn_cast<HLSLBufferDecl>(TheDecl);
-
   // exactly one of these two types should be set
-  assert(((TheVarDecl && !CBufferOrTBuffer) ||
-          (!TheVarDecl && CBufferOrTBuffer)) &&
-         "either TheVarDecl or CBufferOrTBuffer should be set");
-  (void)TheVarDecl;
-  (void)CBufferOrTBuffer;
+  assert(((isa<VarDecl>(TheDecl) && !isa<HLSLBufferDecl>(TheDecl)) ||
+          (!isa<VarDecl>(TheDecl) && isa<HLSLBufferDecl>(TheDecl))) &&
+         "expecting VarDecl or HLSLBufferDecl");
 
   RegisterBindingFlags Flags = HLSLFillRegisterBindingFlags(S, TheDecl);
   assert((int)Flags.Other + (int)Flags.Resource + (int)Flags.Basic +